DESCRIPTION: (Applicant's Description) Cell proliferation is tightly controlled to prevent loss of growth control, a hallmark of cancer. The cell cyclel inhibitor p27kipl, although seldom deleted or mutated, nevertheless plays an important role as tumor suppressor in preventing breast cancer. Decreased p27 protein levels correlate with a more aggressive disease, higher rates of reoccurrence, and poorer long term survival for young breast cancer patients. The clinical observations highlight an important problem that must be addressed by biologists--what are the mechanisms controlling p27 protein levels, and how are they compromised in tumorigenesis? p27 is thought to be regulated mainly by translational or proteolytic mechanisms. We have recently identified the reduction/oxidation (redox) environment as a new way of regulating p27 structure and function. Preliminary results indicate the cell cycle inhibitor p27 can exist in an oxidized or a reduced state. Only reduced p27 functions as an inhibitor of cyclin dependent kinases--the oxidized form of p27 functions solely as a CDK substrate. We and others have proposed that p27 degradation is controlled by phosphorylation, and that the kinase responsible for determining p27 protein levels may be cyclin E-CDK2. Thus, the preliminary data provide a convincing rationale for redox regulation of p27: oxidation not only prevents CDK inhibition, but it may favor p27 phosphorylation and elimination from the cell. The focus of this proposal is to characterize p27 redox regulation in a system of purified proteins. Amino acids(s) involved in redox regulation will be identified by mutagenesis. Chemical agents capable of modulating redox environment will be examined for their ability to switch p27 between inhibitor and substrate roles. A transient transfection procedure will then be used to express wild type and mutant p27 cDNAs in tissue culture cells. p27 redox state and activity against cyclin-CDKs will be examined to identify any differences which may be due to redox regulation. Chemical agents will be examined for their ability to alter redox environment in cells and hence p27 structure and function. The long term objective is to determine whether p27 is redox regulated in normal or aberrant cell physiology, and to ascertain whether targeting p27 redox regulation represents a novel way to intervene in the development of breast cancer.